There's a local proshop here that also does not tie string loops. They refuse for liability reasons, and I don't fault them for that. Because of that I recently learned to do it myself. Here are some references to help you out:

Tie on a string loop

Tie on nocking points

If you're worried about losing your nock position, just take a measurement from either the end of the serving or froma mark on a cam. Then you can remove your nock and know that you can still properly locate the nocking position.